A new video shows the moon dancing around in a celestial version of do-si-doing across the Sun, stopping and seemingly retracing its steps in the other direction.
Except that’s sort of impossible.
According to NASA’s Solar Dynamics Observatory (SDO), the Moon’s maneuvering can be explained by the relative speeds and positions of the Moon, the Sun, and the SDO that, when perfectly aligned, create the trippy optical illusion called “retrograde motion”. Here, a celestial object looks like it is moving backward because of the way that various objects around it move at different speeds and points in the orbit.
When a celestial body passes between a larger body (in this case, the Sun) and an observer (here, the SDO), it creates what astronomers call a transit. The first part of this last transit was captured as the Moon moved from left to right. At a rate of 3 kilometers (1.9 miles) per second, the SDO moved perpendicular to the Sun-Earth Line to eclipse the Moon, which moved at 1 kilometer (0.6 miles) per second. In the second part of the transit, the Moon appeared to pause and rewind. This occurred as the SDO entered the dusk part of its orbit and started to move away from the Moon, almost parallel to the shadow that it was casting through space. When the moon began to move faster than the SDO, it overtook it and thus appeared to move in the opposite direction.
SDO often captures the Moon moving in different directions during lunar transit, but perfect conditions this time gave the dancing illusion. Altogether, the event lasted a total of four hours from 5pm until 9:07pm EST. At its peak, the Moon covered 82 percent of the Sun’s face.
Last September, SDO captured two lunar transits, when the Moon passed in front of the Sun and the Moon appeared to move in one direction and switch back to cross the Moon again. This too was based on the SDO’s perspective as the spacecraft’s orbit caught up and passed the Moon during the first transit.
SDO is an initiative launched by NASA to understand how solar variations influence life on Earth and our technological systems.